C pitfalls and pitfalls Chapter 7. Portability pitfalls 7.10 Free first, then realloc

First release, then reallocate
The C language implementation provides three memory allocation functions for users: malloc, realloc and free.malloc(n) will return a pointer to a newly allocated block of memory that can hold characters of n, which the programmer can use.free will free the memory allocated by malloc by passing the pointer returned by the malloc function as an argument to the free function.realloc needs to pass in the pointer to the allocated memory area of ​​a block of memory and the new size of the memory as parameters, so that the memory area can be adjusted (expanded or reduced) to the new size. This process may involve memory copying.
memory reallocation (reallocation)
This implementation allows a block of memory to be reallocated after it is freed, provided that the memory reallocation operation is performed early enough.(UNIX)
For example, the following code is legal:
free(p);
p = realloc(p, newsize);
Free all elements in a linked list:
for(p = head; p != NULL; p=p->next) {
free((char *)p)
}
Here, we don't have to worry about calling free, which will make p->next invalid.Recommended.(may fail on other systems)